This invention relates to a gas detecting element, particularly to a gas detecting element of semiconductor type which detects a gas based on the variation of the surface resistance of a semiconductor body when the semiconductor body is put into contact with the gas.
There are known two types of gas detecting elements. One is the gas detecting element of contact combustion type, and the other is the gas detecting element of semiconductor type.
The gas detecting element of contact combustion type is comprised of a filament, e.g. platinum filament. When the filament comes into contact with a combustible gas, its temperature rises and its electric resistance becomes higher. Based on this resistance elevation the gas detecting element of this type detects the combustible gas. But, the filament evaporates little by little and becomes thinner and thinner since it is heated to a high temperature for a long time. As a result, the electric resistance of the filament becomes higher. Thus the gas detecting element of contact combustion type is defective in view of gas detection accuracy. To eliminate this drawback, some modifications have been proposed. For example, U.S. Pat. No. 3,092,799 shows the technique of embedding a filament in a body made of a heat-resistant oxide such as alumina and silica and impregnating the outer layer portion of the heat-resistant oxide body with an oxidizing catalyst. The gas detecting element thus modified is, however, not a satisfactory one in view of its gas sensitivity and its gas selection characteristic. Further, it does not remain stable during a long use.
The gas detecting element of semiconductor type is comprised of an oxide semiconductor body. When the oxide semiconductor body cames into contact with a gas, its surface resistance varies. Based on this resistance variation the gas detecting element of this type detects the gas. For example, when a N-type semiconductor body made of ZnO, SnO.sub.2, Fe.sub.2 O.sub.3 or the like comes into contact with a reducing gas, its resistance is lowered. Conversely, when the N-type semiconductor body comes into contact with an oxidizing gas, its resistance is elevated. On the other hand, a P-type semiconductor oxide has its resistance elevated when brought into contact with a reducing gas and lowered when put into contact with an oxidizing gas. The gas selection characteristic, i.e. reactivity with various gases, of such a semiconductor oxide is determined by its surface temperature, surface electron level, porosity and pore size etc. Generally, however, such a semiconductor oxide body alone cannot make a satisfactory gas detecting element since its gas sensitivity and gas selection characteristics are insufficient.
Attempts have been made to elevate the gas sensitivity of an oxide semiconductor body. To achieve this object, it was proposed to impregnate an oxide semiconductor body with a catalyst. But this technique proved defective. Namely, the optimum calcination temperature of an oxide semiconductor and that of a catalyst differ so much that it is extremely difficult to determine the temperature at which to calcine the oxide semiconductor and catalyst together, without diminishing their desirable characteristics. Further, the catalyst is likely to exist as a solid solution in the oxide semiconductor body when it is calcinated together with oxide at a high temperature or while the resultant gas detecting element is used at a high temperature (To elevate its gas sensitivity, the element is preferably heated by a heater to maintain its surface temperature at a few hundred degrees centigrade.). As the catalyst is reformed into a solid solution in the oxide semiconductor body the gas sensitivity of the element is reduced and the aging of the element promoted.